1. Field of the Invention
This invention relates to a picture encoding method and apparatus for encoding a still image or moving pictures using wavelet transform.
2. Description of Related Art
Typical of the conventional representative picture compression systems is the JPEG (Joint Photographic Coding Experts Group) system standardized by ISO (International Organization for Standardization). This JPEG system is a system for compressing and encoding mainly a still picture using DCT (discrete cosine transform). This JPEG is known to give acceptable encoded and decoded pictures on the condition that a larger number of bits is allocated. However, if, in this system, the number of encoding bits is diminished to a more or less extent, block distortion peculiar to DCT becomes outstanding to make subjective deterioration apparent.
Recently, such a system comprising splitting the frequency spectrum of a picture into plural bands by a set of filters combined from plural high-pass and low-pass filters, known as a filter bank, and encoding the picture from band to band, is being researched vividly. In particular, the wavelet encoding, which is free from a defect proper to DCT that block distortion becomes apparent at high compression, is thought to be promising as a new technology to take the place of the DCT.
In encoding moving picture, there are currently known the MPEG-1, MPEG-2 and the MPEG-4 of the MPEG (Moving Picture Image Coding Experets Group) system. Of these, the MPEG-2 is widely used for compressing the so-called DVD (Digital Versatile Disc). In the encoding techniques, used in JPEG and MPEG, encoding control is made from one macroblock to another. It is noted that several 8×8 blocks, each of which is a processing unit for DCT, usually 16×16 blocks, make up one macroblock.
At present, the JPEG system, the MPEG system or the so-called DV (digital video) system is used in a majority of video products, such as electronic still cameras or video movies. These encoding techniques, exemplified by JPEG and MPEG, use DCT as the transform system. Since the above products, employing the wavelet transform as basis, are predicted to be presented to the market, researches towards improving the efficiency of the encoding system are conducted briskly in many research laboratories. In actuality, the JPEG2000, a format expected as the next-generation international standard system for a still picture, may be said to be a successor to JPEG, and is being worked out by ISO/IEC/JTC1SC29/WG1, which is of the same organization as that of JPEG. It is scheduled that recommendations for standardization of the part 1 of the JPEG2000 will be issued in March 2001. In this JPEG 2000, it has been decided that wavelet transform is to be used in place of the DCT of the existing JPEG as the transform system as the basis of picture compression.
Meanwhile, if desired to acquire not only an encoded still picture of high quality but also encoded high-quality moving pictures, it is crucial to solve the following problems:    (i) Since the wavelet transform usually executes transform on the entire picture, it is impossible to perform fine control for each specified area in a picture, such as macro-block based control in DCT of the MPEG or JPEG.    (ii) For overcoming the drawback (i) above, there is such a technique in which a picture is divided into tiles or blocks, each being a rectangle, usually a square, of a specified size, and encoding control is performed separately for each tile which is regarded as a picture. The technique has a defect that, (a) if the tile size is decreased, the encoding efficiency is lowered, and that (b) if the compression ratio is raised, discontinuities between neighboring tiles become outstanding to lower the subjective picture quality significantly.    (iii) In wavelet transform encoding, as in DCT encoding, picture quality control is by quantization control. In general, if the quantization step is increased, the number of bits generated is suppressed, however, the picture quality is deteriorated. Conversely, if the quantization step is lowered, the number of bits generated is increased, however, the picture quality is improved. This quantization control needs to be realized in terms of a specified picture area as a unit, irrespective of whether or not the control is to be used as encoding means for tile-based encoding as in (ii) above.